This invention relates to a brake fluid pressure controller having functions both as an antilock controller and a traction controller.
An antilock controller in the simplest form is known as a fluid return type. It comprises a wheel brake fluid pressure control valve including a discharge valve and is provided in a main fluid line extending from the master cylinder to the wheel brakes, a discharged fluid reservoir for temporarily storing the brake fluid discharged from the discharge valve, and a circulation pump for suctioning brake fluid in the discharged fluid reservoir and returning it to the main fluid line.
If it is desired to add the traction control function to this device, the simplest and most efficient way to do so is to use the circulation pump as a pressure source for the traction control.
In order to use the pump for this purpose, it is necessary to supply brake fluid to the inlet port of the pump. There are several known methods for supplying brake fluid to the pump inlet.
These methods are roughly classified into two types. In one type, a fluid supply line is provided which extends directly from a reservoir of the master cylinder to the pump inlet. In the other type, such a fluid supply line branches from the main fluid line connecting the outlet of the master cylinder to a wheel brake. The latter arrangement is more advantageous because the entire device can be more easily mounted on the wheel and because there is no need to return brake fluid from the inlet of the master cylinder to its reservoir.
In the latter arrangement in which the fluid supply line branches from the main fluid line, it is necessary to provide a traction control changeover valve to check the fluid flow from the point at which the fluid discharged from the pump flows into the main fluid line, toward the master cylinder during traction control, and provide a shutoff valve to check the fluid flow from the fluid supply line toward the discharged fluid reservoir during a normal braking mode (while pressure is being applied to the master cylinder).
One problem with the latter arrangement is that the amount of brake fluid supplied to the pump tends to be limited due to intake resistance at the inlet of the master cylinder, flow resistance in the piping from the master cylinder to the brake fluid pressure controller, and flow resistance in the shutoff valve for closing the fluid supply line during normal braking, so that the brake fluid pressure may not rise quickly at the initial stage of traction control. One way to solve this problem is to provide an intermediate fluid reservoir in the fluid supply line.
The arrangement including such an intermediate fluid reservoir has the following problems.
Such an intermediate fluid reservoir is so adapted that when the fluid pressure in the intermediate fluid reservoir drops below the atmospheric pressure, its volume will decrease, so that the internal fluid pressure is always kept substantially equal to the atmospheric pressure. Upon completion of traction control, its volume increases by the force of a spring or the resilience of a rubber member until it becomes equal to the initial volume.
Thus, if the traction control continues for a long time, brake fluid in the reservoir of the master cylinder may be drawn through the fluid supply line into the intermediate fluid reservoir during traction control because of the tendency of the intermediate reservoir to return it to the high-volume side.
If the brake is applied while the brake fluid in the reservoir of the master cylinder is being suctioned into the intermediate reservoir during traction control, a greater amount of brake fluid than the brake fluid sucked into the brake circuit in an amount corresponding to the stroke of the brake pedal during a normal braking mode will be suctioned into the brake circuit.
If, in this state, the brake pedal is released, the cup seal in the master cylinder (in the case of a conventional master cylinder) or the rubber seal of center valve (in the case of a center-valve type master cylinder) will be gnawed by the side port when passing therethrough.
Such a problem could occur with any type of master cylinder, including the abovementioned conventional type, a center-valve type single master cylinder, conventional+conventional type, center valve+center valve type, or center valve+conventional tandem master cylinder, and other types (e.g. tilt-valve type).